Fiber Bragg grating (FBG) optical filters are used extensively in optical networks for stabilization of semiconductor pump lasers employed in erbium doped optical amplifiers, as well as for wavelength division multiplexing and dispersion compensation of wavelength channels propagating in the optical networks. FBGs also attracted considerable attention for use as sensors in monitoring of civil structures, and for sensing temperature and strain in various environments. Each FBG has a characteristic retro-reflective Bragg resonance at a Bragg resonance wavelength λBr. The Bragg resonance wavelength λBr is dependent on periodicity of the grating within the optical fiber, and on the effective refractive index of the fiber core. The Bragg resonance wavelength λBr is defined by:λBr=2neffΛ  (1)
wherein neff is an effective refractive index for the resonating guided core mode, and Λ is a period of the grating within the waveguide.
FBGs are usually manufactured using high power ultraviolet (UV) laser systems operating in a nanosecond pulse or a continuous wave (cw) regime. Typically, FBGs are generated by exposing a UV-photosensitive core of a germanium doped silica core optical fiber to a spatially modulated UV laser beam, to create permanent refractive index changes in the fiber core. Such a spatially modulated UV beam can be created by using a two-beam interference technique disclosed in U.S. Pat. No. 4,807,950 by Glenn et al., or by using a specialized transmission diffraction grating disclosed in U.S. Pat. No. 5,367,588 by Hill et al. In a Hill et al. device, a phase mask is precision etched in a glass substrate, such as silica, to optimize coupling of incident laser light at a given wavelength into a pair of first diffraction orders, and suppressing coupling into the 0th transmission order. Phase masks are often preferred to bulk interferometers for creation of light interference patterns, because bulk interferometers can be less stable and more difficult to use in production environments. Phase masks are more stable, but have less flexibility for adjusting the period of their resulting light interference patterns. For gratings induced with collimated UV laser light and a phase mask, the grating period Λ is related to mask period Λmask byΛmask=2Λ  (2)
In a typical production environment, an inventory of phase masks is maintained to manufacture gratings with different Bragg resonances. Each phase mask can be very costly. Changing and aligning masks to produce different grating resonances on a production line is a time-consuming and tedious process. It is therefore desirable to have a capability to fabricate gratings with different Bragg resonances from a single phase mask, without having to perform a complicated alignment procedure.
Several techniques have been developed, in which a phase mask having a fixed period Λmask is used to manufacture Bragg gratings having different periods Λ. By way of example, Q. Zhang et al. in a paper entitled “Tuning Bragg wavelength by writing gratings in prestrained fibers”, published in Photonics Technology Letters, vol. 6, no. 7, July 1994, present a technique, in which an optical fiber is exposed to an interference pattern generated by a phase mask, while the optical fiber is under tensile strain. When the strain is relieved after the grating inscription, λBr is shifted to lower wavelengths with respect to a similar inscription performed using the same phase mask in unstrained fiber.
Only a small amount of strain can be tolerated by optical fibers, so the amount of λBr adjustment by approach of Zhang et al. is limited. For Bragg gratings written in standard single-mode germanium-doped telecom fiber, such as Corning SMF-28, with λBr in the 1550 nm telecommunication band, the achievable tuning of λBr is less than 2 nm. A variation on this technique is taught by Couillard et al. in U.S. Pat. No. 6,643,066, where the silica phase mask itself is compressed or stretched in order to vary the resultant period of the fiber grating. Again, the maximal amount of tuning of λBr is approximately 2 nm.
Another technique, presented by J. D. Prohaska et al. in a paper entitled “Magnification of mask fabricated fibre Bragg gratings”, published in Electronics Letters, Vol. 29, no. 18, September 1993, uses illumination of a phase mask with diverging or converging ultraviolet laser beams. Referring to FIG. 1, a Prohaska apparatus 10 is shown. An ultraviolet laser beam 11 propagates through a convex lens 12 having a focal length f. Then, the ultraviolet laser beam 11 propagates through a phase mask 13 and impinges onto a photosensitive optical fiber 14. In operation, the phase mask 13 generates converging diffracted beams 15A and 15B, which create an interference pattern 16 having converging fringes 17. Prohaska teaches that magnification M of the Bragg grating period within the fiber 14 is defined by the following equation:
                    M        =                              f            -            p            -            q                                f            -            p                                              (        3        )            
wherein p is a distance between the convex lens 12 and the phase mask 13, and q is a distance between the phase mask 13 and the fiber 14.
Detrimentally, only small changes in periodicity can be achieved by Prohaska method, for the following reason. Light interference patterns produced at a distance from the phase mask are limited by a spot size and a spatial coherence of the illuminating laser beam. Typical UV sources used for grating inscription are excimer laser systems, which have spatial coherence lengths of less than one millimeter. A small coherence length of the UV laser source results in small values of q, which results in the values of M close to unity.
To produce a magnification effect of any significance, the phase mask to fiber distance needs to be at least several millimeters. This distance is beyond the spatial coherence of most excimer laser UV systems used to inscribe Bragg gratings. Alternative UV systems include frequency doubled argon ion lasers which have better spatial coherence, but sub-millimeter beam sizes. To ensure sufficient overlap of diffracted orders to create an interference pattern, the phase mask to fiber distance cannot be greater than 1 mm in argon ion laser based systems. It is noted that coherent UV beams from a frequency doubled argon ion laser can have their beam diameters expanded, which would allow for a greater phase mask to fiber distance. However, the beam intensity of an expanded beam is much lower. Because the grating strength is linearly dependent on exposure time, a wide low-intensity beam could eventually write a grating after a lengthy exposure. However, long writing times with an expanded frequency-doubled argon ion laser beam might not be practical from a manufacturing standpoint.
Bhatia et al. in U.S. Pat. No. 6,269,208 disclose a method to improve a spatial coherence of an excimer laser UV beam to increase the phase mask to fiber distance, to enhance the magnification effect. Bhatia et al. teach that a coherence length of an excimer laser beam may be increased by spatially filtering the laser beam using an aperture. A convex lens is placed in an optical beam path downstream of the aperture. By varying a distance between the aperture and the convex lens, one can vary a divergence of the beam exiting the convex lens and incident on the phase mask. The spatial filtering increases the spatial coherence of the laser beam, allowing for increased distance between the phase mask and the photosensitive waveguide.
One disadvantage of this approach is that significant writing power of the laser beam is lost as a result of the spatial filtering, while the spatial coherence of the beam is only marginally improved. For a standard excimer laser, the improvement in the spatial coherence length, which defines the maximum distance between the phase mask and fiber, is increased from 0.5 mm to approximately 2 mm. Larger phase mask to fiber distances are needed in order to get a wavelength tunability of the Bragg grating of over 5 nm.
Cole et al. in U.S. Pat. No. 6,072,926 teaches a technique for adjusting the period of a fiber Bragg grating by laterally displacing the phase mask and the UV beam along the fiber axis during the writing process. The phase mask to fiber distance is kept within the spatial coherence length of the UV laser, which is less than 1 mm. The tuning range of the generated Bragg grating for a given phase mask was also limited, approximately 1 nm or less. The adjustment range increases as the width of the collimated UV beam on the phase mask is reduced, however reduced beam size requires that the fiber be directly adjacent the phase mask to ensure sufficient overlap of orders diffracted by the phase mask, to create the required interference pattern. Precision translation stages are needed to translate both the optical fiber and the phase mask with respect to each other.
In all the above cases, practical wavelength tuning will produce wavelength differences in a telecom wavelength band of 1525 nm to 1565 nm of only a few nanometers at best. Painchaud et al. in U.S. Pat. No. 6,501,883 disclosed a grating inscription technique based on a combination of phase mask scanning along the optical fiber taught by Cole, and beam focusing taught be Bhatia. The Painchaud et al. technique is capable of increasing the variation in wavelengths produced from a single phase mask by an order of magnitude, up to 20 nm. Detrimentally, expensive translation and alignment stages are required to precisely control a relative motion between the phase mask and the optical fiber waveguide.
Recently, femtosecond infrared lasers coupled with phase masks have been used to obtain Bragg gratings in optical waveguides not sensitive to ultraviolet light. Mihailov et al. in U.S. Pat. Nos. 6,993,221 and 7,031,571 disclose techniques for fabrication of Bragg grating structures in optical fibers and waveguides using a phase mask illuminated by an ultrafast laser source having a pulse duration of less than 500 fs. The resultant grating structures have a high amplitude of refractive index modulation, over 1×10−3. The induction of index change is much more efficient using femtosecond pulse duration infrared radiation in comparison with continuous wave or nanosecond pulse duration UV radiation.
The approach of illuminating phase masks with non-collimated femtosecond laser beams to arrive at different grating pitches has been demonstrated by Voigtländer et al. in an article entitled “Ultrashort pulse inscription of tailored fiber Bragg gratings with a phase mask and a deformed wavefront”, published in Optical Materials Express, vol. 1, no. 4, pg. 633-642 (2011), and by Song et al. in an article entitled “Tunable Direct Writing of FBGs into a Non-Photosensitive Tm-Doped Fiber Core with an fs Laser and Phase Mask”, published in Chinese Physics Letters vol. 26 no. 9, paper 094204 (2009). In both cases, the non-collimated beam is created by introducing a singlet cylindrical lens before the phase mask. The singlet cylindrical lens has its axis of curvature parallel to the lines of the phase mask.
Although large wavelength shifts of about 40 nm were demonstrated by Song et al., the shift in Bragg resonance is fixed by the focal distance of the cylindrical lens, for a given phase mask to fiber distance. From a production point of view, the phase mask to fiber distance needs to be kept constant, because any change of phase mask to fiber distance results in a change of diffracted order overlap, causing a variation of an overall grating length and, accordingly, resulting in a variation of spectral properties of the grating, such as reflectivity and bandwidth. Therefore, if a grating period is to be changed, the cylindrical lens needs to be changed. This change results in a discrete variation of the grating period, and requires tedious and time-consuming re-alignment of the manufacturing setup.